The invention is concerned with disposable inserts which can be used for turning, boring and milling. In particular, the invention relates to indexable inserts of hexagonal shape, commonly known as trigonal inserts, with a specific chip breaker form.
Disposable indexable inserts are used in many machine tools. They are generally detachably secured to a holder to be used for cutting etc. When the cutting edge is worn during operation, the insert can be removed from the holder, turned such that a new cutting edge is presented to the workpiece and once again secured to the holder. When all the cutting edges have been used, the insert is replaced.
Hexagonal shaped inserts are used in various machine tools such as turning tools, drilling and grooving tools. A known type of hexagonal shaped insert has alternate acute and obtuse angled corners. When a hexagonally shaped insert of this type is used, the two cutting edges making up the obtuse angle are generally applied to the workpiece. An example of a hexagonally shaped insert is shown in U.K. Patent No. 1,359,238.
Hexagonal shaped inserts of the above type can be used in drilling tools. An example of such a use is shown in U.K. Patent Application No. 2000704A. In this arrangement, cutting bits are arranged in a recess at the end of a drill shank. The hexagonal cutting insert has six cutting edges of equal length of which two are in engagement with the workpiece at each time. The two operative cutting edges are those cutting edges which make up one of the obtuse angles of the hexagonal insert. To improve the cutting of the borer, the operative cutting edges each form approximately the same angle to a line parallel to the turning axis of the drill. The working region of the operative cutting edges of each of the inserts overlap as the drill turns so that material is removed from the bore being drilled.
One of the problems with the above type of insert is that because the radial distance of each of the adjacent cutting edges used in the drilling operation is different, unequal forces are applied during the cutting operation. For example, the outer cutting edge of a particular insert has a greater surface speed than the inner cutting edge for a given turning speed of the drill.